NEW ORLEANS (AP) — The federal judge presiding over a trial arising from the nation’s worst offshore oil spill says it could be difficult to determine how much crude spewed into the Gulf of Mexico from BP’s blown-out well in 2010.

U.S. District Judge Carl Barbier noted during a hearing Thursday that there wasn’t a “meter” attached to BP’s Macondo well.

Barbier heard testimony earlier this year for the trial’s first phase about possible causes of the disaster, which killed 11 rig workers. Determining how much oil spilled into the Gulf is a topic for the trial’s second phase, set to begin Sept. 16.

BP PLC claims the federal government overestimated the spill’s size. The government estimated more than 200 million gallons spewed out, including more than 34 million gallons that were collected.